Impacts of Geometry, Symmetry, and Morphology of Nanocast Co3O4

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Impacts of Geometry, Symmetry, and Morphology of Nanocast Co3O4 on Its Catalytic Activity for Water Oxidation Xiaohui Deng, Wolfgang N. Schmidt, and Harun Tüysüz* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz, 45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: Herein, we report a systematic study on the synthesis of ordered mesoporous Co3O4 nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N2 sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co3O4. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co3O4 replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co3O4 replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co3O4 replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co3O4 shows superior activity to the cubically ordered one.

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Cubically ordered mesoporous silica KIT-6 is a well-known template, which has been intensively utilized for nanocasting. It has Ia3d̅ symmetry with two interpenetrating bicontinuous mesopore systems.26,27 Both channel systems are interconnected by micropores. The interconnectivity can be tuned, e.g., by changing the aging temperature during the KIT-6 synthesis. The wall thickness between two channels gets smaller and interconnectivity gets higher when the aging temperature is increased. When KIT-6 aged at 35 °C is used as a template, the replica may show a lower symmetry of I4132 since the degree of interconnectivity is not sufficient to allow the metal oxide to grow in both channel systems.28 In this case, the metal oxide only grows in one channel system in a given volume of the template, which gives rise to bigger pore size and an open subframework.29,30 On the contrary, when KIT-6 aged at high temperature such as 100 or 135 °C was used as template, the replica is more likely to show the same symmetry as the template due to substantial interconnectivity between adjacent channels. Such phenomenon has been reported and explained at length in the literature.27−31 Another prominent silica template for nanocasting is SBA-15, which has a 2D structure with well-ordered hexagonal arrays of unidimensional meso-

here has been an increasing interest in the design of ordered mesoporous transition metal oxides because of their fascinating properties such as high surface areas, large pore volumes, and uniform and narrow pore size distributions, making them highly valuable model systems for various research areas. Diffusion limitations that are typically observed for microporous zeolites, in particularly in catalysis, could be overcome with the advent of ordered mesoporous materials at the beginning of 1990s.1,2 The history, synthesis, and applications of ordered mesoporous metal oxides (OMMs) have been covered in several excellent review articles.3−11 The synthesis methodology of this class of materials can be divided into two major branches, i.e., soft templating and hard templating routes. Compared with soft templating, hard templating, also named nanocasting, has been applied to numerous types of transition metal oxides with precisely controlled composition and high crystallinity, properties which are difficult to achieve with conventional synthesis pathways.10,12−25 In a typical nanocasting process, an ordered mesoporous template (silica or carbon) is first filled with a metal oxide precursor by wet impregnation or solid−solid mixing. Then thermal treatment is applied to crystallize metal oxides in the mesopores of the hard template. Finally, the template is removed by leaching (silica) or calcination (carbon) and the product is obtained as the negative replica of template. For simplicity, the term replica will be used for negative replica in the following. © 2014 American Chemical Society

Received: June 26, 2014 Revised: October 13, 2014 Published: October 20, 2014 6127

dx.doi.org/10.1021/cm5023163 | Chem. Mater. 2014, 26, 6127−6134

Chemistry of Materials

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followed by stirring at 35 °C for 24 h. Subsequently, the mixture was aged in a box oven preheated to 100 °C for another 24h in a closed container. The solid product was filtered, dried at 90 °C and finally calcined in air at 550 °C for 6h. Preparation of Hexagonal Ordered Mesoporous Silica Template. SBA-15 was synthesized according to existing literature under acidic conditions using a triblock copolymer (Pluronic P123, EO20PO70EO20, Sigma-Aldrich) as structure directing agent.32 First, 27.8 g of P-123 was dissolved in 504 g of deionized water and 15.5 g of HCl (37%). After a clear solution was obtained, TEOS was quickly added into the mixture by stirring at 35 °C. The mixture was further stirred at 35 °C for 24 h, followed by a hydrothermal treatment at 100 °C for 24 h in a closed container. The solid white product was filtered without washing, dried overnight at 90 °C, and subsequently calcined at 550 °C for 6 h. Preparation of Ordered Mesoporous Co3O4. Co(NO3)2·6H2O was used as metal precursor in both cases. Briefly, a defined amount of silica template (KIT-6 or SBA-15) was dispersed in 0.8 M Co(NO3)2· 6H2O ethanol solution and kept under stirring for 1 h at room temperature. Then it was dried at 60 °C overnight without filtration and subsequently calcined. The amount of precursor used was adjusted to the intended loading amount and pore volume of the silica template. The latter was measured by N2 sorption prior to the use of the silica. In one impregnation cycle, 5% of the pore volume was filled with Co3O4, assuming it has the same density as bulk Co3O4 (6.11 g/ cm3). For each loading (5, 10, 15, and 20%), different numbers of impregnation cycles were applied by starting from same batch of silica. One, two, three and four impregnation cycles were applied for 5%, 10%, 15% and 20% of total pore volume filling, respectively. After each impregnation cycle, the samples were calcined at 200 °C for 4h to decompose the cobalt nitrate precursor to Co3O4. After the final impregnation was applied, the respective sample was calcined at 500 °C for 6 h with a plateau at 250 °C for 4 h. The silica was removed by 2 M hot NaOH solution for 48 h. Electrochemical Measurement. Electrocatalytic water oxidation experiments were carried out in a three-electrode configuration (Model: AFMSRCE, PINE Research Instrumentation) with Ag/AgCl as reference electrode and a Pt wire as counter electrode. Aqueous KOH solutions with various concentrations were used as electrolyte and argon was bubbled through then solution to remove the oxygen during the measurement. Working electrodes were fabricated by depositing target materials on glassy carbon (GC) electrodes (PINE, 5 mm diameter, area of 0.196 cm2). The surface of the GC electrodes was polished with Al2O3 suspension (5 and 0.25 μm, Allied High Tech Products, INC.) before use. After that, 5 μL of catalyst/ethanol suspension was dropped on the GC surface after sonification for 20 min (catalyst loading ∼0.12 mg/cm2 for all the samples). The electrode was then dried at room temperature. Subsequently, 5 μL of 0.25 wt % Nafion solution was dropped on the GC as binding agent. Finally the electrode was dried under light irradiation. All current versus potential curves were measured in a rotating disc electrode (RDE) configuration with a sweep rate of 20 mV/s at 2000 rpm. The electrochemical measurements were carried out at least on two GC working electrodes to check the reproducibility and their average was taken into account. Potentials are reported vs Ag/AgCl reference electrode and overpotentials are calculated as Eoverpotential = EAg/AgCl + 0.197 V + 0.0591pH − 1.23 V. The current density reported in this work was normalized to the geometric surface area of glassy carbon electrode. Characterization. All the chemicals and reagents were purchased from Sigma-Aldrich and used without further purification. Wide-angle XRD patterns collected at room temperature were recorded on a Stoe STADI P theta/theta diffractometer in Bragg−Brentano geometry equipped with a secondary flat graphite monochromator (Cu Kα1/2 radiation). The measured patterns were evaluated qualitatively by comparison with entries from the ICDD PDF-2 powder pattern database or with calculated patterns using literature structure data. SAXS data were obtained with a Anton Paar SAXSess Kratky camera (width of detection area, 3 mm; exposure time, 1 s; number of frames, 600). TEM images of samples were obtained with an H-7100 electron

pores.32 Replicas in the form of nanowires can be easily produced from this type of hard template.13,31,33 There are several parameters that influence a successful replication such as nature of the template, type of the replica precursor, and various parameters of the templating procedure itself. The latter include the impregnation and drying method, the precursor decomposition method (oxidation, reduction), the heating rate, the calcination temperature and time, and the template removal method.3,30,31,34 Some other less essential parameters were reported to affect the structure of the replicas as well, including the type of the drying container and the fact whether calcination was performed with an open or a closed crucible.35 By controlling the above parameters, we could control geometry, symmetry, surface area, pore volume, domain size, crystallite and pore size, wall thickness, and crystallinity of the replica to some extent, which makes this process highly attractive for the design of numerous ordered mesoporous compounds. Cobalt oxide has attracted attention in the field of solar energy utilization because of its catalytic activity in water oxidation.36 That reaction is essential for artificial photosynthesis since electrons must be transferred from oxygen anions in water to protons to form hydrogen or liquid fuel.37−39 Being more abundant and cheaper compared to more commonly used Ir and Ru based catalyst, Co-based materials are quite promising with respect to onset potentials and reaction rates in electrochemical and photochemical applications.40,41 In particular, mesostructured Co3O4 has gained considerable attention for energy applications due to its high surface area and structural stability.39,42−44 It has been reported that mesoporous Co3O4 has superior activities to its nanosized counterpart with a higher activity and better stability for water oxidation.39,42,45 The main advantage of this material is its porous network structure that prevents the aggregation and sintering, which is a key advantage in comparison with nanoparticles. In addition, most of the morphology and shape controlled nanostructured materials are prepared by using a surfactant as capping agent that can influence the catalytic performance of the materials. Clean, surfactant free surface of highly crystalline nanocast materials is another appealing benefit. Thus, it seems rewarding to investigate the effect of geometry, symmetry, and textural parameters of the nanocast Co3O4 on water oxidation with the aim to develop electrocatalysts with superior energy efficiency. Herein, we investigated in details the loading amount effect on the structure evolution of Co3O4 nanocast from 3D cubically ordered KIT-6 and 2D hexagonally ordered SBA-15 (aged at 100 °C). Although studies have been conducted in the past to elucidate effects of metal precursor loading on framework interconnectivity,31 no study has ever been focused on very low loading amounts to the best of our knowledge. In addition, a study on the electrocatalytic activities toward water oxidation has been performed.

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EXPERIMENTAL SECTION

Preparation of Cubically Ordered Mesoporous Silica Template. Synthesis of KIT-6 was conducted as described before.26 Briefly, triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, EO20PO70EO20, Sigma-Aldrich) was employed as structure directing agent. 13.5 g Pluronic P123 was dissolved in a mixture of deionized water (487.5 g) and concentrated HCl (37%, 26.1 g). After a homogeneous solution was formed, n-butanol (13.5 g) was added at 35 °C. After 1 h of stirring, TEOS (29.0 g) was added to the solution as the silica source, 6128



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aging temperature such as 40 °C is employed as hard template, the obtained replicas possess a lower symmetry due to the low degree of the interconnectivity between the mesopore channels of the silica template. However, if the silica template has been synthesized at higher aging temperature such as 100 °C and consequently processes a high degree of interconnectivity, the produced material would be an exact negative replica of the silica template that has the same symmetry. Our investigation now shows that by reducing the loading amount the symmetry of the nanocast Co3O4 could also be modified. The change of symmetry goes along with changes of the mesopore system and textural parameter of the replica. N2 sorption isotherms and respective pore size distributions are depicted in Figure 2. All samples show type IV isotherms,

microscope (100 kV) from Hitachi. N2 sorption isotherms of the Co3O4 were measured with an Micromeritics ASAP 2010 adsorption analyzer at 77 K. Prior to the measurements, the samples were degassed at 200 °C for 10 h. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.97. BET surface areas were determined from the relative pressure range between 0.06 and 0.2. Pore size distributions were calculated by the BJH method from the desorption branches of the isotherms. XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, analyzer pass energy of 40 eV was applied. The hybrid mode was used as lens mode. The base pressure during the experiment in the analysis chamber was 4 × 10−7 Pa. To account for charging effects, all spectra have been referred to C 1s at 284.5 eV. The Co3O4 sample for XPS measurements after long-term catalysis was prepared in the following way: 3 mg c-Co3O4-5 was first dispersed in ethanol and after thorough sonication, the catalyst ink was dropped on FTO glass and calcined at 250 °C in air. The reaction was conducted under a current of 10 mA for 2 h in 0.1 M KOH. Finally, the samples were scratched from FTO and collected for XPS measurements.

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RESULTS AND DISCUSSION Cubically ordered mesoporous Co3O4 (donated as c-Co3O4) with increasing amount of Co3O4 loading in the silica template (5, 10, 15, and 20% of the total pore volume of silica host) were prepared via nanocasting using cubically ordered mesoporous silica KIT-6 that had been aged at 100 °C as hard template. The sample series is labeled as c-Co3O4-X, where c denotes the cubic geometry and X represents the loading amount. TEM images and the small-angle X-ray scattering (SAXS) profile of the silica template are shown in Figure S1 (see the Supporting Information). The SAXS data of c-Co3O4 as shown in Figure 1 Figure 2. N2 sorption isotherms and pore size distributions (inset) of Co3O4 samples replicated from KIT-6 with varying loading amount. An offset of 50 cm3/g is applied to the isotherms.

indicative for mesoporous materials. However, as seen in Figure 2, the sorption isotherm of c-Co3O4-5 shows a broad range in which capillary condensation takes place, extending from p/p0 = 0.75 to saturation pressure, indicating a higher porosity and relatively large pore sizes. When the pore filling is increased to 10%, a clear hysteresis loop between p/p0 = 0.45 and p/p0 = 0.8 can be observed and capillary condensation at higher relative pressure is reduced. Further increasing the pore filling has negligible effect on the shape of the hysteresis loop whereas no further capillary condensation is observed at higher relative pressures. From the pore size distributions it can be seen that when the loading amount is higher than 10%, only a narrow monomodal pore size distribution centered at 4 nm is present and this is equal to the wall thickness of silica hard template, which indicates the Co3O4 was present in both channel systems of the KIT-6. For c-Co3O4-5 a bimodal pore size distribution is observed. A second pore system with a pore size distribution centered at about 13 nm is associated with the structural peculiarities of template if one considers formation of the replica in only one pore system of KIT-6. The bigger pore sizes indicate that the Co3O4 was not deposited in equal amounts in adjacent mesopore systems of the template. All samples have BET surface areas of about 105 m2/g. In contrast to this, the pore volumes of the samples, in particular those of c-Co3O4-5 and c-Co3O4-10 significantly differ from each other. The total pore volume of c-Co3O4-5 (0.47 cm3/g) is substantially larger than that of c-Co3O4-10 (0.20 cm3/g), further confirming a

Figure 1. SAXS profiles of Co3O4 samples replicated from KIT-6 with varying loading amount.

indicate that all samples with a loading amounts of more than 5% show (211) and (220) reflections, which are characteristic of replicas from KIT-6 aged at 100 °C (denoted as KIT-6-100) with a cubically ordered structure with Ia3̅d symmetry. Unpredictably, with the lowest loading (namely c-Co3O4-5), an additional peak that corresponds to (110) reflection becomes more obvious and that gives rise to a lower symmetry of I4132.28 This symmetry is not common for replicas from KIT-6-100 because the interconnectivity of KIT-6-100 is believed to be sufficient for producing the replica with original symmetry. Earlier studies showed that when KIT-6 with lower 6129



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more open pore system of c-Co3O4-5c. Increasing the total loading amount further decreases the pore volume of the replicas to a small extent. The SAXS and N2 sorption data are further supported by TEM images. TEM micrographs at lower magnification which show a more representative fraction of respective materials are presented in the Supporting Information (Figure S2). TEM images with higher magnification are presented in Figure 3. As

Figure 3. TEM micrographs of ordered mesoporous Co3O4 samples replicated from KIT-6 with varying loading amounts of (a) 5, (b) 10, (c) 15, and (d) 20%. Scale bars are 50 nm.

shown, all c-Co3O4 samples possess an ordered mesoporous structure. Changes of morphologies and differences of the particle sizes can be easily perceived from the TEM micrographs. c-Co3O4-5 has an average domain size of about 100 nm. Increasing the Co3O4 loading in the template results in substantially larger particles, as c-Co3O4-10 has particle sizes of around 190 nm and for c-Co3O4-20 the particle sizes reach about 500 nm, as can be seen from the Supporting Information (Figure S2). A substantial number of particles with an uncoupled open framework and larger pores are observed for c-Co3O4-5 (Figure 3a), which is responsible for the larger pore volume and the bimodal pore size distribution as observed by N2 sorption. With increasing pore filling of the silica template, the domain sizes of the obtained mesoporous Co3O4 gradually increases and nearly all particles are formed in interpenetrating framework. All data indicate that in some parts of the silica template growth of Co3O4 takes place only in some of the channels when a low amount of inorganic precursor is applied. The amount of the precursor seems not enough to continuously fill the micropores which interconnect adjacent mesopores. This would result in a partial disintegration of the interpenetrating Co3O4 frameworks. Increasing the number of loading cycles could enhance the degree of interconnectivity of the Co3O4 mesostructure, thus resulting in larger particle sizes and interpenetrating mesostructures. No bulk Co3O4 is observed in all cases, confirming the successful replication of the silica templates. SBA-15 aged at 100 °C was employed as template to synthesize hexagonally ordered mesoporous Co3O4 (labeled as h-Co3O4-X, where h and X indicate the hexagonal structure and the loading amount). As can be seen from Figure 4a, the SAXS profiles of h-Co3O4 materials all show (100), (110) reflections,

Figure 4. (a) SAXS profiles and (b) N2 sorption isotherms and pore size distributions (inset) of Co3O4 samples replicated from SBA-15 with varying loading amount. An offset of 50 cm3/g is applied to the isotherms.

which are related to the hexagonal structure with p6mm symmetry. However, due to a relatively low loading amount, hCo3O4-5 shows less pronounced reflections, indicating a slightly lower degree of the ordering. Because SBA-15 has only parallel arrays of unidirectionsal channels, the symmetry of the Co3O4 replicas remains the same for all samples. Increasing the loading amount to more than 10% has no effect on the structural ordering. N2 sorption isotherms and pore size distributions are presented in Figure 4b. The total pore volumes of h-Co3O4 follow the same trend as observed for c-Co3O4, i.e., decrease with higher loading. The hysteresis loops at lower relative pressure become more pronounced when the pore filling is 10% or more, indicating the presence of more ordered structure. TEM images clearly show the effect of loading amount on the degree of ordering of the obtained mesostructured cobalt oxide. As shown in Figure 5, nanowires are separated from each other by a constant distance equal to the wall thickness of the SBA-15 template, which confirms the successful replication of mesoporous structure. At the lowest loading (h-Co3O4-5), disordered arrangements of nanowires with relatively short lengths are observed whereas large bundles of parallel Co3O4 nanowires are obtained at higher loadings. SBA-15 has mesoporous channels that are connected with each other 6130



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linking are observed for h-Co3O4-5. Increasing the pore filling to 10% or higher value results in exact negative replica of SBA15 (h-Co3O4-10, -15, -20). Additional loading does not change the connectivity and morphology of the nanowires visibly but boosts the lengths of the nanowires. Because Co3O4 has been extensively reported as water oxidation catalyst, a systematic study on the electrocatalytic activity of the prepared materials was performed. The linear sweeps of cubically and hexagonally ordered Co3O4 in 0.1 M KOH solution (pH 13) are shown in Figure 7. It has been reported that the surface of Co3O4 changes partially to CoOOH at the rest potential,46 and prior to water oxidation reaction it is further oxidized to CoO2 which corresponds to the anodic peak around E = 0.55 V (vs Ag/AgCl) as shown in Figure 7. In the case of 3D ordered c-Co3O4, a clear trend is observed with c-Co3O4-5 being the most active one. The electrocatalytic activity decreases with the increase of loading amount. The electrochemical data was taken on multiple electrodes and averages were taken (the polarization curves of c-Co3O4-5 and c-Co3O4-20 from multiple electrodes are shown in Figure S4 in the Supporting Information to demonstrate the reproducibility). Figure S5 in the Supporting Information displays the Tafel plots of c-Co3O4-5, c-Co3O4-20 and hCo3O4-20 derived from Figure 7. As can be expected, similar Tafel slopes in the range of 86−96 mV/dec indicates the relatively sluggish kinetics. It has been reported recently that nanocast Co3O4 shows a Tafel slope of ∼70 mV/dec under pH 14.47 To have a better comparison with benchmarking cobaltbased electrocatalyst, we also tested c-Co3O4-5 in 1 M KOH electrolyte, and an overpotential close to 410 mV was required to obtain a current density of 10 mA/cm2.48 The result is shown in Figure S6 (see the Supporting Information) and the Tafel slope is calculated to be 59 mV/s, which is reasonable comparing with other reported values.40,47,49 However, as shown in Figure S5a in the Supporting Information, it is worth pointing out that although the deviations from linearity in the Tafel plots originate nearly from the same overpotential for both samples, the development of current in the mixed-control (activation control and mass-transport control) region shows obvious difference. Compared with c-Co3O4-20, in the case of c-Co3O4-5 with an open framework and larger pore volume, the overpotential needed for certain current density is relatively closer to the value which is required by Tafel equation (as indicated by the yellow dash line), indicating the mass-transport limitation may be less dominant. The performance dependence on loading amounts can be explained as an effect of structural

Figure 5. TEM micrographs of ordered mesoporous Co3O4 samples replicated from SBA-15 with varying loading amounts of (a) 5, (b) 10, (c) 15, and (d) 20%. Scale bars are 200 nm.

through micropores. When the loading amount is low, the amount of Co3O4 that grows in the micropores of the template is not sufficient to form highly interconnected nanowires in the mesopores of the SBA-15. Increasing the loading amount from 5 to 10% results in a higher degree of interconnectivity and consequently higher ordering of the Co3O4 nanowires. Further increasing the loading to 15 or 20% does not change the structural ordering noticeably, although it increases the length of the nanowires. This can also be observed from the micrograph surveys shown in Figure S3 in the Supporting Information. The above results and discussion are summarized schematically in Figure 6. Filling the channels of KIT-6 with low amounts of Co3O4 (5%) results in an incomplete replica (cCo3O4-5) with lower symmetry. The material has bimodal pore size distribution, larger pore volume and a more open subframework structure. Increasing the loading amount to 10% or more results in negative replicas with monomodal pore size distribution and interpenetrating Co3O4 frameworks with the same symmetry as the silica template. An increase of particle size and a decrease in the total pore volume are observed with increasing loading. The same trends apply for replication from SBA-15. A loading of 5% is not sufficient to fabricate an array of hexagonally ordered mesoporous nanowires. Instead, loose aggregates of nanowires with low degree of

Figure 6. A schematic illustration of replication of 3D (c-Co3O4) and 2D (h-Co3O4) Co3O4 from KIT-6 and SBA-15 silica templates with varying loading amounts (5, 10, 15, 20%). 6131



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electrocatalytic activity of the material substantially. Diffusion seems to present fewer problems for h-Co3O4 replicas. Table 1 Table 1. Textural Parameters and Electrocatalytic Activities (overpotential and current density) of 3D (c-Co3O4) and 2D (h-Co3O4) Replicas with Varying Loading Amounts (5, 10, 15, 20%)

c-Co3O4 -5 c-Co3O4 -10 c-Co3O4 -15 c-Co3O4 -20 h-Co3O4 -5 h-Co3O4 -10 h-Co3O4 -15 h-Co3O4 -20

specific surface area (m2/g)

total pore volume (cm3/g)

overpotential (mV) at 10 mA/cm2

current density (mA/cm2) at 1 V vs Ag/AgCl

104

0.47

496

30.1

108

0.20

508

29.2

107

0.16

513

26.8

107

0.15

537

25.8

109

0.35

515

26.4

110

0.27

500

32.6

108

0.24

501

32.0

110

0.22

502

32.4

summarizes the textural parameters and electrocatalytic activities of materials discussed in this work. As presented, among all 2D and 3D replicas, c-Co3O4-5 requires the lowest overpotential of 496 mV at 10 mA/cm2, whereas c-Co3O4-20 requires the highest one of 537 mV. When the voltage reached 1 V, h-Co3O4-10, h-Co3O4-15, and h-Co3O4-20 samples showed the highest current densities ∼32 mA/cm2 among all samples. The reason responsible for the activity difference might be the physical contact of the sample with the surface of the working electrode. One would expect a better contact for hexagonally ordered mesoporous replica because of their fewer dimensions, and this might influence charge transfer and eventually the material’s activity. Moreover, the stability of most active samples from both templates is tested under constant applied voltage (0.8 V vs Ag/AgCl), and satisfying stability is obtained in both cases (see Figure S7 in the Supporting Information). Only slight deactivation which has also been shown for bare Co3O4 elsewhere is observed.41 Because Co3O4 may undergo surface change after prolonged electrolysis, samples after catalyzing long-term water oxidation were collected and investigated with X-ray photoelectron spectroscopy. As can be seen from Figure S8 in the Supporting Information, the oxidation states of surface cobalt remains unchanged after applied external bias is terminated. However, a slight shift to lower binding energies after long-term test can be observed from the detailed comparison of Co 2p photoemission. This can be due to the conductivity drop or the formation of small amorphous domains on the surface once the electrode is off electrochemical working conditions. Deeper investigation would be required and is beyond the scope of this work.

Figure 7. Oxygen evolution currents of (a) 3D c-Co3O4 and (b) 2D hCo3O4 with varying loading amounts dispersed on glassy carbon electrode in 0.1 M KOH electrolyte (catalyst loading ∼0.12 mg/cm2 for all the samples).

parameters since all the samples exhibit very similar surface areas. As discussed before, c-Co3O4-5 has a more open structure and much larger pore volume compared with the other c-Co3O4 samples. The samples obtained from 10, 15, and 20% pore filling have more or less the same surface areas, but their particle sizes and pore volumes are slight different from each other. Decreasing activity of the samples is related to increasing domain size and decreasing total porosity. The latter reduces the accessibility of matrix and may hamper diffusion of reactant and products in the mesopore system during the reaction. Electrocatalytic investigation on hexagonally ordered mesoporous h-Co3O4 indicates an opposite trend and the electrocatalytic activity benefits from increased loading amounts, leaving h-Co3O4-5 less active than h-Co3O4-10, whereas no substantial change is observed when more impregnation cycles are applied. As discussed above, h-Co3O4-5 has a lower degree of interconnectivity and ordering than h-Co3O4-10, -15, or -20. Although h-Co3O4-5 has higher porosity, in the case of hexagonally ordered samples, the effect of ordering seems more dominant than that of pore volume. As has been shown, with similar mesostructure and surface area, increasing the loading amount from 10 to 20% slightly decreases the total pore volume of the replicas. However, the latter does not affect the

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CONCLUSION We have demonstrated that the symmetry of Co3O4 replica, which is nanocast from KIT-6 can be tuned by changing the loading amount of the cobalt precursor. Low loading such as 5% is not sufficient to generate a high degree of the interconnectivity between mesopores. As a consequence, the 6132



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symmetry of the replica that has bimodal pore size distribution is lower than silica template and it has larger pore volume and smaller domain sizes. Increasing the loading to 10%, results in an exact negative replica of the silica hard template. Further increase of the pore filling does not change the symmetry of the replica; however, it influences the morphologies and textural parameters of the replicas. Their domain sizes increase with loading, whereas the total pore volume decreases. In the case of replicas that are nanocast from SBA-15, low loading such as 5% is not sufficient to form highly ordered mesostructures because of a low degree of interconnectivity as observed also for nanocasting from KIT-6. Increasing the total loading to 10% is sufficient to obtain a highly ordered structure and further increasing the loading does not change the structure noticeably but longer nanowires are obtained. Electrochemical studies showed that there is a significant effect of the mesostructure on the catalytic activity of Co3O4 toward water oxidation. Hexagonally well-ordered 2D replicas result in higher current density at higher voltage in comparison to less ordered nanowires and their 3D counterparts. Among the 3D replicas, highest activity was observed for c-Co3O4-5 with lower symmetry, more open subframework and larger pore volume. The electrocatalytic activities of 3D replicas decrease with increasing domain sizes and decreasing porosities, indicating that for this type of materials restricted diffusion might be a crucial factor. Our results indicated that the alteration of the loading amount of precursor could influence the structure, morphology and textural parameters of replica, which would eventually affect the catalytic performance of materials. The methodology that is presented here can be used to create other mesostructured transition metal oxides with controllable geometry, symmetry, and morphology that might be interesting for various catalytic applications.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of KIT-6 and SBA-15 silica hard templates (SAXS, N2 sorption, TEM). Lower magnification TEM images of 2D and 3D nanocast Co3O4. Electrochemical data obtained from multiple electrodes for c-Co3O4-5 and c-Co3O4-20. Electrochemical measurement conducted at 1 M KOH for cCo3O4-5 and stability test of c-Co3O4-5 and c-Co3O4-20. Tafel plots of selected samples. XPS spectrum of Co3O4 before and after long-term electrochemical water oxidation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Our work was supported by the Max Planck Society and the research network SusChemSys, funded by the State Government of Northrhine Westphalia (Ziel2-NRW funded by Europäischer Fonds für regionale Entwicklung). We thank Dr. C. Weidenthaler for XPS experiments and helpful discussion. 6133



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Impacts of Geometry, Symmetry, and Morphology of Nanocast Co3O4

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